Plant Mineral Nutrients and Vitamins
Role of minerals:
When this planet earth took its origin about 3.8 billion years ago (nearly) along with the sun, it had the same elements what it has today. It is possible new elements have been created during the origin of solar system for the atoms and molecules have gone through fusion and splitting. Among 113 elements or so, hardly few elements have been used for creating life. Carbon, Hydrogen, Oxygen, Nitrogen are mostly important and mainly responsible for the formation of organic compounds. Some more elements like phosphorus, sulphur, potassium, calcium, and other metal elements are used either as the structural components or as catalysts for the reactions. Since then, life has chosen only few more elements and its compounds as nutrients for all biological activities.
Chemical analysis of the plant body reveals the presence of the following elements, i.e., C, O, H, N, K, Na, Ca, Fe, Cu, Mo, Mn, Mg, P, Bo, I, SO, Si, Se etc. The concentration of individual components varies from plant to plant, but some are found in almost every kind of plants. Those nutrient elements that are absolutely required for the normal growth and development, without which plants exhibit diseased symptoms, are called essential nutrients. The diseased symptoms due to the deficiency of particular elements can be corrected by supplanting the deficient element in adequate quantities. As a result, diseased plants become normal and complete their reproductive cycle.
The essential elements are further classified into two groups. This is based on the quantitative requirements for healthy growth of the plants. Some elements are required in large quantities and others are required in extremely small amounts but essential for the normal growth of the plants. The former is called macro elements and the latter is called as micro or trace elements. Besides the above said elements there are few more elements in the plant body but their absence does not cause symptomatology or disease, such are called non-essential mineral nutrients. Macro nutrients- require in large amounts C, N, Ca, K, Mg, P, N and S: Micro nutrients- require in small amounts; Fe, Mn, Cu, Zn, Mo, Bo, Cl, and Co.
Role of essentials minerals:
1. Important elements like C, O, H & N and phosphorus go into the formation of bulk of the organic molecules of all living organisms including plants. And such simple organic molecules in turn organize into macromolecules, like carbohydrates, nucleic acids, proteins, lipids, pigments and amino acids and others. Many of the above said components act as structural, functional or storage compounds of cells, tissues, and organs. A bulk of it goes into plant non-living matter, such as the cell walls and the wood.
2. Some of the inorganic components act as catalysts either in Free State or bound to enzymes or other molecules.
3. The presence of these elements determines the pH and the chemical potential of the cellular protoplasm. The functional activities of the protoplasm depend upon the pH of the cell.
4. Some of the elements influence the permeability of other ions ad thus changes the osmotic concentration and turgidity of the cell or they felicitate the absorption of ions or antagonize it.
The role of individual elements though characteristic, the totality of a function or the development of the plant body depends upon the interaction of different elements. In this chapter, a brief account of some important functions and the symptomatology due to deficiency of them has been described. As carbon, oxygen, and hydrogen are the major organic constituents of every living organism, and as they are available mostly from air and water, their pervasive role need not be emphasized. So, there is no separate discussion about the role of carbon, hydrogen and oxygen. The role of other essential elements has been discussed.
Nitrogen:
More than 78% of atmosphere is made up of N2. Though all living organisms are submerged in an ocean of gaseous N2, not all plants are capable of utilizing this form of N2 directly from air. Only a few micro-organisms are endowed with a potentiality to use this from of inert N2. Inspite of it, the main source of N2 to plants is available in the soil either in the form of NO3, NO2, NH4 or organic nitrogen.
Nitrogen is an important component of amino acids, proteins, nucleic acids, pigments, amides, hormones, alkaloids and many other minor compounds. For that matter, N with C, H & O2 can be considered as the mainstay of life. Without nitrogen, life ceases to exist.
DIFFICIENCY SYMPTOMS
1. Growth is retarded due to lack of cell division and cell enlargement thus shows stunted growth.
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2. Chlorophyll content is lowered and leave turn yellow in color.
3. It affects respiration, photosynthesis, amino acid synthesis, protein synthesis and nucleic acid metabolism.
4. Cellular proteins get depleted.
5. Induces dormancy and senescence, shedding of leaves, reduction of flowering are the common symptoms. Fruits remain immature or remain small even if they are mature.
PHOSPHORUS
It is available in soil in the form of metallic phosphates like calcium phosphate, magnesium phosphate or as ammonium phosphates, etc. The ionic forms of phosphates are readily absorbed by the roots.
The most important role of phosphates is in the formation of bonds in ATP, GTP, CTP TTP, UTP and other triphosphate nucleotides. ATP is the main source of energy for all living organism. Phosphate is required not only at the time of energy trapping during photosynthesis, but also during ATP synthesis by biological oxidation. Most of the phosphorylation of histone and non histone proteins help in the regulation of gene expression. Cyclic AMP and cyclic GMP are the other important components which act as a second messenger next to hormones. Many organic compounds of phosphor go into membrane structures and also felicitate the translocation of many components.
Symptoms of deficiency:
1. Growth activity in terms of meristematic division is reduced or totally inhibited.
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2. Leaves look dark green in color.
3. Stems remain slender.
4. Fruits ripen slowly.
5. Premature fall of leaves is common.
6. Dormancy is prolonged.
SULPHUR
The source of sulphur is soil, where it is available in the form of sulphates such as CaSO4, MgSO4, Ammonium sulphate, etc.
The characteristic odor in onion and garlic is due to the presence of sulphates in volatile oils found in them. Sulphates are also found in some important membranes as lipid components. The most important role of sulphur is being present in amino acids like cysteine and methionine. These amino acids play a pivotal role in 3-D structure of proteins and also act as the active sites for many enzymes. Sulphur is also an important constituent of some vitamins like, biotin, thiamine, and co-enzyme.
Symptoms:
1. Leaves remain small and appear light green in color or yellowish green.
2. Chlorosis is another common symptom.
3. Fruit formation is reduced to a great extent.
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Boron
Boron is available in soil in the form of borates. Elements form of boron is very rare to be founding the soil.
Boron is found to be antagonistic to the uptake of potassium and a few other cations, but favors the absorption of calcium. Boron is very effective in inducing reproductive structures in Marchantia and inducing germination of pollen tubes. It is also implicated in taking part in fat metabolism, photosynthesis, phosphate metabolism, etc. But the most significant role played by boron is in the translocation of photosynthates, particularly the transportation of sucrose across membrane. Use of radioactive 14C sucrose in translocation studies supports this view.
Symptoms:
1. Death of the stem tip and root tips is a significant symptom.
2. Heart rot of sugar beat, browning of cauli flower, top sickness of tobacco, hardness in citrus fruits is common.
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3. Decreases the production of flowers.
4. Abnormal tillering, cutting, brittleness symptoms are not uncommon.
Potassium
Soil particles are rich in potassium compounds in the form of glauconite, mica, fluorspar, etc. These are made available to plants by weathering processes. These elements are fixed in soil clay particles and available only by ion exchange mechanism.
It plays an important role in the synthesis of proteins in vivo or invitro. The concentration effect is manifested by its ability in controlling the translation of different types of mRNA’s. It also activities many enzymes and enzyme factors. The ions are highly mobile in the plant tissues. High concentration of are found in meristematic tissues. It regulates turgidity of the cell, opening and closing of the stomata. Sleeping movements of leaves in many plants is due to the change in the flux of these ions. It is also involved in activating enzymes involved as respiration, photosynthesis, nucleic acid synthesis of chlorophyll.
Symptoms:
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Mottled Chlorosis in leaves, followed by necrosis is common symptoms. Stunted growth scorched margin of leaf curling of leaf tips and margins are characteristic symptoms of deficiency.
Calcium:
Soil rich in lime stones or chalk rocks contain greater amounts of calcium. In the soil it is fixed to anion clay micelles and it is available by hydroxyl ion exchange mechanism.
Calcium is found in the middle walls of plants cells as calcium pectate. This acts as the cementing material between the cells. It is also found in the plasma membrane as one of the lecithin complexes. Calcium activates Ca2+ dependent ATPase system. It is found in the chromosomes and maintains its integrity. Besides, calcium also plays a very important role in the polymerization of microtubules and action mediated contractile processes. It is also known to be involved in carbohydrate translocation. Many enzymes are also known to be activated by calcium.
Symptoms:
Chlorosis at the margin of the leaves is a common occurrence. Pedicels and petioles collapse due to its deficiency. Leaves show curled appearance. Roots develop poorly and meristematic regions break down, it provides strength as in human beings’ skeletons. Sometimes, chromosomes undergo fragmentation in its absence.
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Magnesium
Magnesium is available in the soil as carbonates. It can be released from fixed state by cation exchange mechanism. It leaches out very easily.
Magnesium is an important component of chlorophyll; it plays a similar role as of iron in heme group. Magnesium is very essential for the integrity of functional ribosomes. Many enzymes which involve ATPs are Mg2+ dependent. RNA polymerase, DNA polymerase and protein synthesis requires mg2+ as an activator.
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Symptoms: Deficiency causes vein Chlorosis, yellowing, pigmentation, necrosis brittleness in leaves, etc.
Iron
Soil is rich in the oxides of iron. It is available as ferric or ferrous oxides. The red-brown coloration of the soil is due to the presence of iron in high quantities.
Iron being a component of many electron transport and oxidation enzymes; it plays a significant role in the production of ATP during oxidative phosphorylation. It is an important activator for many enzymes like ferrodoxin, Catalase or peroxidase, ferrichrome, hematin and leghemoglobin. Iron regulates the synthesis of chlorophyll.
Symptoms:
Extensive Chlorosis of leaves but veins remain green yellow, scorching at the margins and tips, sometimes the entire leaf gets bleached when this element is deficient.
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Molybdenum:
It is rich in mineral oils and coal ashes. But in soils, it is never deficient and plants require traces of this mineral.
It is an important component of nitrogenase and nitrate reductase enzyme. These enzymes are involved in the conversion of NO3 or N2 into utilizable form of N2. It acts as an activator of some phosphotases and dehydrogenase. It also acts as the cofactor in the synthesis of ascorbic acid.
Symptoms: Deficiency causes light yellow Chlorosis, mottling of leaves. In certain grass members seed / fruit setting is inhibited.
Copper:
Copper is never a deficient element in soils. But in copper fields not all plants grow, because of its toxicity under high concentrations. It is available as copper nitrate or copper sulphate.
It is an important component of cytochrome oxidase found in the electron transport chain. Copper containing plastocyanin is another component of electron transport system in photosynthetic organelles. Copper is found in some enzyme systems like ascorbic acid oxidase, phenolases, etc.
Symptoms:
Retardation of growth that includes both vegetative and reproductive. Burning of leaves at the margin, resetting and chlorosis are few symptoms due to deficiency; grain development is severely affected in the case of deficiency of this element.
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Zinc:
It is not lacking in soils, but it is found in extremely small quantities. Zinc sulphates and zinc oxides are few forms available for plants.
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Zinc acts as the prosthetic group of many enzymes like carboxypeptidase, lactic dehydrogenase, alcohol dehydrogenase, RNA polymerase and alkaline phosphotase, etc. It is also involved in nitrogen metabolism.
Symptoms: Necrotic spots, vein chlorosis, mottling of leaves, resetting of apples, citrus and it also causes little leaf disease by its absence.
Manganese:
Oxides and sulphates of manganese are found in the soil. It is available in the soil either in an oxidized or reduced state.
Manganese is an important component of photosynthetic unit II, where it is bound to Z-protein. This complex is absolutely essential for oxidation of water in releasing of oxygen and electrons. It acts as a cofactor for enzymes like RNA polymerase, Malic dehydrogenase and hydroxyl aminases. It is also involved in the destruction of a plant hormone called auxin.
Symptoms
Deficiency causes total crop failure, it develops grey speckle disease in oat leaves; brown necrosis of cotyledons in pea, chloroplasts become yellow; vacuolated and finally disintegrate.
Besides the above-mentioned essential nutrients, some elements like chlorine, cobalt, aluminum, silicon and others are also found to play some roles. But their deficiency does into cause any severe diseases as the others do. However, the role of cobalt is very well known because it is an important component of cobalamin, and vitamin (B12). It plays an important role in nitrogen fixation particularly in symbiotic bacteria. It is also involved in the synthesis of a few amino acids through enzyme activation.
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Nutrient solutions and culture methods:
Knowledge of nutrition in the management of crop plants has led to the formulation of balanced nutrient solutions. Using this composition, it is possible to grow and maintain the plants under artificial conditions. This knowledge has been fully exploited in advanced countries for growing horticultural plants as well as vegetables and fruit plants on a commercial scale.
There are many well known nutrient media available and each of them have to be supplemented or slightly modified depending upon the kind of plants grown.
Hoagland’s Nutrient Media:
1. Sand Culture:
Sand is first acid treated and washed and steamed under high pressure for about 30 minutes for about 30 minutes or so. Then the sand is filled into containers like plugged pots or glass jars. After adjusting the pH of the solution, the nutrients are supplied in adequate quantities so that sand should not be water logged. To such containers plants can be transplanted and grown. Weekly supplements of nutrient solution of known quantity are essential. This method is not used because of some draw backs for it is difficult to control the pH changes and to measure the amount of nutrients lost or required. But this provides a natural media for the growth of roots. If properly used sand can substitute Agar under in vitro tissue culture.
1. Solution Culture or Hydroponics and Aeroponics:
Solution culture has become very popular, for the methods involved are easy. Though the initial investment is a little high the output is good. Clean glass containers of any suitable size can be used. Plants can be grown in glass containers filled with the nutrient media. Meshes are provided for the support of plants. For aeration air bubbling should be maintained. Weekly supplements of salts and water is essential. Even required hormones can be supplied this way.
Hydroponics has been successfully used in cultivating commercial crops like tomato and capsicum and other vegetable crops. In contrast to hydroponics under certain laboratory conditions another method is used called Aeroponics. This method involves having the seedlings in the air and mineral nutrients are sprayed as mist. The roots adsorb the minerals from the moist air. This method is very expensive.
2. Invitro Cultures:
The same nutrient solutions can be used to culture cells and tissues into organs and plants under aseptic conditions. The nutrients further require the addition of proper vitamins and plant hormones. Using this method, it is possible to grow a complete plant from any single living cell from any part of the plant body, provided the cell possesses a nucleus. Tissue culture methods have as greater application in horticultural, agricultural industry and pharmaceutical industries. Now a days even some vaccines are produced by using transgenic plants on large scale.
ROLE OF VITAMINS
Vitamins are a class of organic compounds. They are absolutely required for the maintenance of healthy life processes of both plants and animals including bacteria. Though they are required in small amounts, their capability of sustenance and their ability to perform biochemical functions is remarkable. The history of vitamins is one of the most important chapters in the field of medicine. People in olden days, knew that night blindness can be overcome or cured buy eating liver or using liver extracts. It is not until the British sailors on seven seas suffered from scurvy, that people realized the requirement of lime juice as a supplement in their diet. In fact, it was Casimir Funk who demonstrated that the extracts from rice husk, was able to cure the disease of Beriberi prevalent in human beings. Realizing the importance of some compounds which had amines so vital to the human body they were called as ‘Vitamins’. Since then, remarkable progress has been made in this field. A large number of vitamins have been discovered, their structure and functions have been elucidated. It is paradoxical that the highly evolved human being is incapable of synthesizing the required vitamins, it is by the quirk of nature, the plants are endowed with the ability to synthesize all the required vitamins and also capable of supplying to other animals. Because of plants capacity to synthesize almost all vitamins in its body, it is difficult to obtain data on the symptoms due to deficiency of vitamins. However, the knowledge about vitamins deficiency in plants has come from the invitro studies of root cultures where roots don’t grow without the addition of some vitamins this indicates that roots cannot synthesize some vitamins that are needed for their growth. Nonetheless, the knowledge about vitamins role in cellular metabolism has come mostly from animal studies. In this text a brief account of the source, structure and function of vitamins has been discussed. The study of vitamins is very important in understanding the biological processes. Based on the solubility, vitamins have been grouped into water soluble vitamins and fat-soluble vitamins. Fat soluble vitamins are A, D.E & K and the rest are water soluble. Most of the vitamins have been found to act as coenzymes and some act as growth regulators.
VITAMIN B GROUPS
Thiamine (Vitamin B1): Deficiency of this causes beriberi in human beings. In plants this vitamin is found in large quantities in actively growing regions and also in the husks of cereal grains. It is synthesized in leaves in the presence of light and then translocates to other regions preferably to growing regions.
Thiamine consists of a substituted pyrimidine ring. It forms a bridge with another substituted thiozole by a methyl bridge. It exists as a free from and also as bound form called thiamine pyrophosphates.
Thiamine pyrophosphate acts as the coenzyme for enzymes such as pyruvate decarboxylase and keto gluteraldehyde decarboxylase reactions where CO2 is removed.
Riboflavin (Vitamin B2): The deficiency of this vitamin causes a disease in human beings called. This vitamin is synthesized in plant leaves. Even micro-organisms produce them.
This yellow pigment is derived from isoalloxanthine. There are two types of riboflavins called riboflavin mononucleotide and riboflavin dinucleotide. They act as the prosthetic group of some important respiratory enzymes, ex., succinic dehydrogenase and NADPH dehydrogenase etc. They are involved in electron transport and transfer of hydrogen groups. It is suspected that this compound is involved in absorbing light at 445 mm and cause destruction of IAA and brings about phototropic curvature in stem tips. Delbruck calls this pigment as ‘Cryptochromes’.
Nicotinic acid (Niacin): In human beings, the deficiency of this vitamin causes a disease called pellagra and black tongue in dogs. In plants, it is found in leaves and green stems. It requires light for its synthesis. For the development of roots this compound is absolutely required. As roots cannot synthesize niacin and its derivates, it is imported from aerial leaves. It is believed that it is synthesized from a precursor called tryptophan, which is also a precursor for IAA.
The term nicotinic acid is used because it is a component of a toxic alkaloid called nicotine found in tobacco leaves. It also exists as an amide called nicotinamide which is linked to a ribose sugar; hence it is called as the derivative of a ribose pyrimidine. There are two important complexes called nicotinamide dinucleotide (NAD) and nicotinamide dinucleotide phosphate (NADP).
These pyrimidine nucleotides function as coenzymes for many oxido-reductases which are involved in respiration, fatty acid metabolism, glucose monophosphate shunt, photosynthesis, amino acid synthesis and other such reactions. It is loosely bound to the apoenzyme part and involved in the transfer of hydrogen group and electrons.
Pentothenic Acid: It is found in significant quantities in aleurone layers of rice, wheat and other cereals. Synthesis of this component takes place in all parts of the plant body.
It is a nine-carbon amino carboxyl compound. Always it is linked to ribosyl adenine diphosphate at one end and to B mercaptoethylamine at the other. This complex is called coenzyme. This vitamin is involved in trans-acetylation reaction of pyruvate dehydrogenase and keto-glyceraldehyde dehydrogenase reactions. It is also involved in the synthesis of acetyl choline and citrate. Pentothenic acid has been implicated in photoperiodic reactions in plants. It plays an important role in the synthesis and oxidation of fatty acids.
Biotin: This vitamin is distributed in all parts of the plant body. Biotin consists of imidazole ring fused with thiophene inactivated by a protein called avidin, found in the white of the egg (raw). Heating destroys this inhibitor. Again, this vitamin acts as the coenzyme covalently bound to some carboxylase enzymes involved in gluconeogenesis.
Vitamin B6 complex: This is made up of a group of pyridoxine and pyridoxal compounds like pyridoxine, pyridoxal, pyridoxal amine, pyridoxal phosphate and pyridoxal amine phosphate.
They are distributed in all parts of the plant body like leaves, stem, roots, seeds, fruits, etc. The active part B6 co enzymes are pyridoxal phosphate and pyridoxalamine phosphate. They are involved in amino acid synthesis by transamination reactions. This is also an important growth factor in plants particularly for tissue cultures for rooting.
Folic Acid: As this acidic compound is derived from the leaves, it is called folic acid. First, it was isolated from spinach leaves and now it is known that it is found distributed in all parts of the plant body. Its deficiency causes various kinds of anemia in human beings.
It is made up of substituted pteridine p-amino benzoic acid and glutamic acid, all together called pteroglutamic acid. The coenzyme form of folic acid is the reduced folate called tetrahdrofolate.
Folic acid is involved in the transfer of hydroxy-methyl group of one amino acid to the other. It actually acts as a kind of shuttle between compounds. This is a very important coenzyme in amino acid metabolism, and also in the synthesis of nitrogen bases required for the nucleotides. In humans it causes tongue boils and blisters
Lipoic Acid: This vitamin is also called as thio-acetic acid. Again, it is found distributed in all the parts of the plant holds. This combines with lysine to form dihydrolipoic acid. As it has two sulphide groups, it is involved in oxidation reduction reactions. This is a very important coenzyme and it is responsible for oxidative decarboxylation reactions of pyruvate and other ketoacids.
Vitamin B12: Vitamin B12 is used to cure pernicious anemia. This is also found in the plants; particularly leguminous root nodules. Structurally, it is very complex and contains cobalt in the middle of coring a ring and it is called Cyanacobalamin; because it contains cyanide group. It acts as coenzyme. This coenzyme is synthesized in symbiotic Rhizobium bacteria. It plays an important role in nitrogenase enzymatic reaction in fixing inorganic N2 into NH3.
Vitamin C (Ascorbic Acid): It is found in all the parts of the plant body but lemon, capsicum and other citrus fruits contain large amounts. The deficiency of this vitamin causes scurvy disease in human beings. Vitamin C acts as a strong reducing agent. It is involved in enzymatic hydroxylation of proline to hydroxyproline which is an important component of extension found in cell walls. It is also involved in cyclic oxidation reduction reactions.
Myo – inositol: A cyclic sugar alcohol called Myo-inositol is an important component of the plant body. It acts as a growth factor in plants and used in plant tissue cultures. Myo-inositol is an important component of membrane Phosphoglycerides. It is also involved signal transduction pathway.
FAT SOLUBLE VITAMINS
Vitamin A: Deficiency of vitamin A causes night blindness in human beings. Though it is synthesized as carotenes in the plant body, particularly in green leaves its role is not known in plant metabolism. From animal experiments, it is now known that it plays an important role in the eyes as an important component of Rhodopsin. In some bacteria like halo-bacteria, this vitamin plays an important role in hydrogen production. It is also known to be involved in growth, spermatogenesis and oogenesis.
Vitamin K: This fat-soluble vitamin is found in plant leaves and other photosynthetic structures, particularly, in chloroplasts. It is involved in cyclic and non cyclic electron transport system. Nothing is known about its other functions in plants. It is also involved in antiport transport of Na ions. But in animals, it is very important in blood clotting. However, the role of other vitamins or such compounds is not clearly known in plants.